In recent years, the techniques used in wavelength division multiplexing (WDM) transmission systems have made remarkable progress. WDM is a technique for transmitting multiple pieces of information by propagating plural wavelengths (or frequencies) of light through a single transmission line, with each wavelength of light carrying a different signal (Note: the term “light” used in this specification includes electromagnetic waves). This technique requires an optical multiplexer and an optical demultiplexer, or wavelength filters, for mixing multiple wavelengths of light at the inlet of the transmission line and then separating the mixed light into each wavelength of light at the outlet. An example of conventional demultiplexers is arrayed waveguide grating. However, to adequately decrease the loss of light, arrayed waveguide gratings currently used are somewhat oversized, as large as roughly several square centimeters.
To increase the capacity of the transmission system and reduce the size of the devices used in it, developments of multiplexers, demultiplexers and wavelength filters using photonic crystals are underway. A photonic crystal is a functional material having a cyclic distribution of refractive index, which provides a band structure with respect to the energy of light. This device is particularly featured in that it has an energy region (called the photonic bandgap) that does not allow the propagation of light. Introduction of an appropriate defect into the distribution of refractive index in the photonic crystal creates an energy level (called the defect level) due to the defect within the photonic bandgap. This allows only a specific wavelength of light having an energy corresponding to the defect level to exist within the wavelength range corresponding to the energy levels included in the photonic bandgap. Forming a linear defect in the crystal provides a waveguide, and forming a point-like defect in the crystal provides a resonator. The shape of the defect determines a wavelength, called the resonance wavelength, at which the resonance of light takes place.
Non-Patent Document 1 discloses the result of a computer simulation of a photonic crystal composed of infinitely long cylindrical elements made of a high refractive index material and arranged in a square lattice pattern. This construction allows light to be controlled by the photonic bandgap within a plane parallel to the square lattice. However, it does not enable the control of light in the direction perpendicular to the aforementioned plane. Photonic crystals having such a construction are impractical.
Patent Document 1 discloses a photonic crystal having a plate-shaped body in which plural areas having a refractive index different from that of the body (called the “modified refractive index area” hereinafter) are cyclically arranged to create a cyclic distribution of refractive index. This construction can control light within the body because a photonic bandgap is present within the plane of the body and the difference in refractive index between the body and the surrounding air confines light within the body in the direction perpendicular to the body. In this construction, a waveguide is formed by eliminating the modified refractive index areas along a line ([0025], FIG. 1), and a point-like defect is formed by eliminating the modified refractive index areas within a point-like region ([0029], FIG. 1). As an embodiment, Patent Document 1 shows a two-dimensional photonic crystal having modified refractive index areas, each consisting of a cylindrical hole, cyclically arranged in a triangular lattice pattern, where the diameter of one of the cylindrical holes located in the proximity of the waveguide is increased to be a point-like defect.
[Non-Patent Document 1] S. Fan et al., “Channel Drop Tunneling through Localized States”, Physical Review Letters, (US), American Physical Society, 1998, vol. 80, pp. 960-963
[Patent Document 1] Japanese Unexamined Patent Publication No. 2001-272555 ([0025], [0029], FIG. 1)
This type of two-dimensional photonic crystal can function as a demultiplexer for separating a ray of light whose wavelength equals to the resonance wavelength of the point-like defect from the light including plural wavelengths superimposed and propagating through the waveguide, and for emitting the light through the point-like defect to the outside. It can also function as a multiplexer that introduces, from the outside of the crystal, a ray of light whose wavelength equals to the resonance wavelength of the point-like defect into superimposed light propagating through the waveguide. Thus, one and the same two-dimensional photonic crystal can function as a multiplexer and as a demultiplexer. Such a two-dimensional photonic crystal is called the “multiplexer/demultiplexer” in this specification. Creating plural point-like defects having different shapes in the proximity of the waveguide provides a multiplexer/demultiplexer in which each point-like defect multiplexes or demultiplexes a ray of light having a different wavelength. In the case where the plural wavelengths of light each carries a different signal, it is possible to extract a specific signal from the transmission line (i.e. the waveguide) with the demultiplexer or introduce a specific signal into the transmission line with the multiplexer.
In the above-described multiplexer/demultiplexer, the point-like defect multiplexes or demultiplexes not only the ray of light having its resonance wavelength, λ0, but also other rays of light included within a certain wavelength range around the resonance wavelength λ0 by certain percentages. In the case of the above-described conventional two-dimensional photonic crystal multiplexer/demultiplexer, the multiplex/demultiplex spectrum takes the form of a Lorenz function around the resonance wavelength λ0, as shown in FIG. 1. A multiplex/demultiplex spectrum expressed by a Lorenz function has a sharp peak; the value of the multiplex/demultiplex spectrum rapidly falls as the distance from the resonance wavelength λ0 increases, and it forms a long tail as the distance becomes much larger. Such a multiplex/demultiplex spectrum expressed by a Lorenz function is accompanied by two problems to be solved with respect to the multiplexing/demultiplexing operations.
The first problem results from the sharpness of the peak of the multiplex/demultiplex spectrum. An aged deterioration or a temperature change in the waveguide 66 have the same transmission wavelength band because they are identically shaped. The difference in cycle between the forbidden band zones 63 and 64 produces a wavelength band 75 that is included within the transmission wavelength band 73 of the waveguide in the forbidden band zone 63 but not within the transmission wavelength band 74 of the waveguide in the forbidden band zone 64. The two cycles can be appropriately determined so that the resonance wavelength of the point-like defects 67 and 68 falls within the wavelength band 75. In this construction, the light having the resonance wavelength and propagating through the input waveguide 65 is reflected at the boundary between the forbidden band zones 63 and 64. A ray of light haveing the resonance wavelength that has passed by the point-like defects 67 and 68 without being introduced into them is reflected back and introduced into the point-like defects 67 and 68. Thus, the demultiplexing efficiency is enhanced. Similarly, if a demultiplexed ray of light is introduced through the point-like defects 67 and 68 into the output waveguide 66, the light is reflected at the boundary between the forbidden band zones 63 and 64 and extracted from only one end of the output waveguide 66. This also contributes to the enhancement of the demultiplexing efficiency. system used may cause an error in the wavelength of light propagating through the waveguide or in the resonance wavelength of the multiplexer/demultiplexer. This will cause a discrepancy δλ between the resonance wavelength λ0 of the point-like defect (i.e. the peak wavelength of the multiplex/demultiplex spectrum) and the wavelength λ1 of the light propagating through the waveguide. As can be understood from FIG. 1, even a very small discrepancy will make the value of the multiplex/demultiplex spectrum at λ1 considerably smaller than at λ0. This means that even a slight change in the wavelength can deteriorate the multiplexing/demultiplexing efficiency if the multiplex/demultiplex spectrum is expressed by a Lorenz function.
The second problem results from the long tail of the multiplex/demultiplex spectrum. Presence of such a long tail will allow rays of light having wavelengths far from λ0 to be undesirably mixed, thus causing a noise. Also, the tail may overlap the wavelength of the signal of an adjacent channel, causing a crosstalk between the two signals.